This invention relates to a portable self-contained test instrument for identifying whether an optical fiber is live, that is, whether light is propagating along the fiber.
For repair and branching purposes, telecommunications workers working with fiber optic cables must distinguish between live (i.e. traffic carrying) and spare fibers. The act of identifying a live fiber must not be such as to overly attenuate or distort the propagating signal and nor must the intrusion be such as to operate any diagnostic alarms used to monitor the fiber or cable integrity.
Our co-pending patent application in the name of Brian S. KAWASAKI et al, U.S. Ser. No. 666,266, filed Oct. 29, 1984, entitled FIBER OPTIC COUPLER, describes a device for measuring the light propagating in a fiber by introducing a bend into the fiber and detecting the light emitted at the bend with a photodetector. The photodetector is connected into a level detection circuit which includes a visual read-out means from which the level of light emitted at the bend can be read. The detection device is used in association with a light launching device when effecting a splice or connection between contiguous fibers. Light is directed into one fiber upstream of the splice or connection site using the local launch device and is detected using the local detection device at a location immediately downstream of the site. The two fiber ends are moved relative to one another until the light level measured at the local detection unit is maximized. In order to maximize light input and output at the local launch and detection devices respectively, the radius of curvature at the bend in the local launch and detection devices is made relatively small, of the order of 2.8 millimeters, and the light source and photodetectors in the respective launch and detection devices are constrained to critical positions in relation to the fiber bend since the input and output light respectively is confined to a narrow beam. The arrangement of elements within the local detection unit described in the above-mentioned co-pending patent application in the name of Brian S. KAWASAKI et al, entitled FIBER OPTIC COUPLER does not lend itself to incorporation within a portable and easily usable test instrument for detecting merely the presence or absence of light within an optical fiber. Also, the induced loss in the fiber due to use of the above device is well above the threshold for triggering diagnostic alarms.
A further patent concerned with the emission of light from a bend within an optical fiber is U.S. Pat. No. 4,270,839 (Cross). The intention of the assembly described in this patent is as an optical fiber signal tap. The arrangement depends on positioning the fiber accurately in a pair of light pipes which can be incrementally moved to displace their axes. Initially the fiber extends along the axes of the two light pipes which are positioned contiguously but with ends spaced from one another. Then as one light pipe is moved so that its axis is displaced from the axis of the other light pipe, light is scattered from the small intervening section of fiber and enters the downstream light pipe. Again this arrangement is not adapted to permit rapid testing of live fibers.